Having gotten a little fed up with manually re-registering MCE files when MedCtrRO.exe fails, I have created a new command-line utility to automate the task.
Of course, I can't warranty any of this stuff, but if you find it useful, feel free to use it. If you run this and still cannot get things working on your machine, please let me know! Please include your %windir%\MedCtrOC.log file, %windir%\ehome\ehshell.crash (if it exists), and if you're experiencing EPG problems, a zipped/RARed copy of your %ALLUSERSPROFILE%\Application Data\Microsoft\eHome\EPG folder.
MceRepair 0.1 (alpha) is posted at http://www.pegasi.com/MceRepair_0.1.zip [obsolete, use 0.2 below]
EDIT: A newer version, 0.2, is now available at http://www.pegasi.com/MceRepair_0.2.zip